On the origin of tropospheric O₃ over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange

This study investigates the origin of a commonly observed feature in the O₃ profiles: mid tropospheric O₃ maxima (300--500 hPa) over the tropical Indian Ocean. A comparison and analysis of model simulations, using a 3-D global climate-chemistry model, and measured O₃ profiles from the INDOEX campaign is presented. European Centre for Medium-Range Weather Forecast (ECMWF) meteorological analyses have been assimilated into the 3-D model to represent actual meteorology. The model realistically simulates the observed mid-tropospheric O₃ maxima. The analysis of the model simulations shows that the major source of the mid-tropospheric O₃maxima is advection of polluted air masses from continental biomass burning areas over Africa, with generally only a small contribution of stratospheric O₃. Previous studies hinted at stratosphere-troposphere exchange (STE) along the subtropical jet (STJ) as the primary source of the mid-tropospheric O₃ maxima over the Indian Ocean. Analysis of the model simulations shows that the mechanism causing the mid-tropospheric transport of African biomass burning pollution and stratospheric air masses are frontal zones or waves passing along the subtropical jets, causing advection of tropical air masses in the prefrontal (equatorward) zone. Furthermore, the frontal zones or waves also cause STE at the poleward side of the STJ. The model simulations also indicate that the contribution of STE in general is minor compared to advection and in situ tropospheric production of O₃ for the mid-tropospheric O₃ budget over the Indian Ocean region

Diurnal ozone cycle in the tropical and subtropical marine boundary layer

A conceptual analysis of diurnal ozone (O3 ) changes in the marine boundary layer (MBL) is presented. Such changes are most pronounced downwind of O3 sources in tropical and subtropical latitudes, and during summer at higher latitudes. Previously, it has been assumed that daytime photochemical O3 loss, and nighttime replenishment through entrainment from the relatively O3 -rich free troposphere, explains the diurnal O3 cycle. We show, however, that in a net O3 -destruction environment (low NOx ) this diurnal cycle can be explained by photochemistry and advection, which establish a horizontal O3 gradient that is typical for the MBL. We support this hypothesis firstly by calculations with a conceptual 1-D advection-diffusion model, and secondly by simulations with an interactive 3-D chemistry-transport model. The results are in good agreement with observations, for example, in the Indian Ocean Experiment (INDOEX)

Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves

Many fast supernova remnant shocks show spectra dominated by Balmer lines. The H$\alpha$ profiles have a narrow component explained by direct excitations and a thermally Doppler broadened component due to atoms that undergo charge exchange in the post-shock region. However, the standard model does not take into account the cosmic-ray shock precursor, which compresses and accelerates plasma ahead of the shock. In strong precursors with sufficiently high densities, the processes of charge exchange, excitation and ionization will affect the widths of both narrow and broad line components. Moreover, the difference in velocity between the neutrals and the precursor plasma gives rise to frictional heating due to charge exchange and ionization in the precursor. In extreme cases, all neutrals can be ionized by the precursor. In this paper we compute the ion and electron heating for a wide range of shock parameters, along with the velocity distribution of the neutrals that reach the shock. Our calculations predict very large narrow component widths for some shocks with efficient acceleration, along with changes in the broad- to-narrow intensity ratio used as a diagnostic for the electron-ion temperature ratio. Balmer lines may therefore provide a unique diagnostic of precursor properties. We show that heating by neutrals in the precursor can account for the observed H$\alpha$ narrow component widths, and that the acceleration efficiency is modest in most Balmer line shocks observed thus far.Comment: 9 pages, 3 figure

Étude comparative de la vitesse de décomposition de H2O2 et de l'atrazine par les systèmes Fe(III)/H2O2, Cu(II)/H2O2 et Fe(III)/Cu(II)/H2O2

Cette étude a eu pour objectif de comparer les vitesses de décomposition du peroxyde d'hydrogène et d'oxydation de l'atrazine par les systèmes catalytiques Fe(III)/H2O2, Cu(II)/H2O2, et Fe(III)/Cu(II)/H2O2. Les expériences ont été réalisées à pH 3,0, à une température de 25,0 (± 0,2) °C, en milieu perchlorate, en présence et en absence d'oxygène dissous. L'étude comparative a confirmé que les vitesses de décomposition de H2O2 et d'oxydation de l'atrazine sont beaucoup plus lentes en présence de Cu(II) qu'en présence de Fe(III) et l'addition de Cu(II) augmente l'efficacité du système Fe(III)/H2O2. Pour nos conditions expérimentales ([composé organique]o < 1 µM) les expériences de cinétique compétitive, réalisées avec des solutions aqueuses contenant trois composés organiques (atrazine, 1,2,4-trichlorobenzène, 2,5-dichloronitrobenzène), ont montré que le radical hydroxyle représente la principale espèce responsable de l'oxydation des composés organiques. Les résultats ont également mis en évidence la formation très rapide d'un composé entre Cu(II) et H2O2 (étude spectrophotométrique) et ont montré l'importance de la concentration en oxygène dissous sur les vitesses globales de décomposition de H2O2 et de l'atrazine par les systèmes Cu(II)/H2O2 et Fe(III)/Cu(II)/H2O2.Toxic and refractory organic pollutants in industrial wastewater can be degraded by advanced oxidation processes (AOPs) alone, or in combination with physico-chemical and biological processes. Of these oxidation methods, Fenton's reagent (Fe(II)/H2O2) and Fenton-like reagents (Fe(III)/H2O2, Mn+ or Mn+1 /H2O2) are effective oxidants of large variety of organic pollutants.The mechanism of decomposition of H2O2 and of oxidation of organic solutes by Fenton's and Fenton-like reactions has been the subject of numerous studies. However, there are still many uncertainties as to the nature of the oxidant species formed and the rate constants of elementary reactions (Table 1).Our recent studies carried out in HClO4 /NaClO4 solutions and in the presence of very low concentrations of organic solutes (atrazine, 1,2,4-trichlorobenzene; concentration < 3 µM) have shown that the reaction of Fe(II) with H2O2 leads to the formation of two intermediates and that the overall initiation step (reaction 1, Table 1) at pH < 3.5 leads to the formation of OH radical (Gallard et al., 1998a). Other work with different organic compounds and higher concentrations of organic solutes indicates that the intermediates (Fe(II)-hydroperoxy complexes, ferrous ion) might also oxidize organic compounds. Ferric ion can also catalyze the decomposition of H2O2. The mechanism is initiated by the formation of two Fe(III)-peroxy complexes at pH < 3.5 (reaction 2a, Table 1) followed by their slow decomposition into Fe(II) and HO2·/O2·- (reaction 2b, Table 1) (Gallard et al., 1999; De Laat and Gallard, 1999; Gallard and De Laat, 1999).The formation of intermediates (complexes, cupryl ion) has also been postulated for the catalytic decomposition of H2O2 by Cu(II). Depending on the experimental conditions (nature and concentrations of organic solutes, pH,…), the degradation of organic compounds might be attributed to the hydroxyl radical (reaction 1, Table 1) or to other species like the cupryl ion (Cu(III)). Production of Cu(III) by reaction of OH· with Cu(II) has also been demonstrated by pulse radiolysis experiments. Kinetic data indicate that the rate of decomposition of H2O2 and the rate of oxidation of organic compounds are faster with Fe(III)/H2O2 than with Cu(II)/H2O2 and that Cu(II) can improve the efficiency of the Fe(III)/H2O2 process.The present study has been undertaken in order to compare the rates of decomposition of H2O2 and the rates of oxidation of atrazine by Fe(III)/H2O2, Cu(II)/H2O2 and Fe(III)/Cu(II)/H2O2 under identical conditions. These conditions (pH 3.0, I=0.1 M, [Atrazine]o < 1 µM) were the same as those used in previous studies of the Fe(II)/H2O2 and Fe(III)/H2O2 systems.Experiments were carried out in MilliQ water, in the dark, at 25.0 (± 0.2) °C, pH 3.0, ionic strength (I) of 0.1 M, in the presence and in the absence of dissolved oxygen. pH and I were adjusted with perchloric acid and sodium perchlorate. The concentrations of hydrogen peroxide ([H2O2]o ≤ 10 mM) and of atrazine ([atrazine]o ≤ 1 µM) were determined iodometrically and by HPLC, respectively.In the absence of organic solutes, experimental results have shown that the rate of decomposition of H2O2 is faster with Fe(III) than with Cu(II) (Figure 2). In agreement with previous data (De Laat and Gallard, 1999), the initial rate of decomposition of H2O2 by Fe(III) can be described by a pseudo first-order kinetic law with respect to H2O2, and dissolved oxygen (0-1 mM) has no effect on the rate of decomposition. For the Cu(II)/H2O2 system, our spectrophotometric data (Figure 1) gave evidence that the decomposition of H2O2 by Cu(II) goes through the formation of an intermediate which might be a Cu(II)-hydroperoxy complex and which absorbs in the region 350-600 nm. Furthermore, the rate of decomposition of H2O2 by Cu(II) does not follow a first-order kinetic law and is affected by the concentration of dissolved oxygen (Figures 2 et 3).As far as the oxidation of atrazine is concerned, a preliminary study of the oxidation of solutions containing atrazine, 1,2,4 trichlorobenzene and 2,5 dichloronitrobenzene in very dilute aqueous solutions ([organic solutes]o < 3 µM) has been conducted at pH 3.0. Experimental results showed that the relative rates of decomposition of organic solutes by Fe(III)/H2O2, Fe(II)/H2O2 and Cu(II)/H2O2 were identical and could be described by the competitive kinetic expression (Figure 4). These data suggest that the oxidation of the organic solutes by the three systems of oxidation tested can be attributed to a unique oxidant species, the hydroxyl radical, under our experimental conditions.The rate of oxidation of atrazine by Cu(II)/H2O2 was found to be much slower than by Fe(III)/H2O2 (Figure 5), to be dependent on the concentrations of reactants ([Cu(II)]o, [H2O2]o Figure 6) and to decrease in the presence of dissolved oxygen (Figure 7). These data confirm that the rate of decomposition of H2O2 by Cu(II), and as a consequence, the rate of production of OH radicals by Cu(II)/H2O2, are much slower than by Fe(III)/H2O2. In addition, a fraction of Cu(I) may be oxidized by dissolved oxygen and this reaction, which competes with the reaction of Cu(I) with H2O2, may also decrease the rate of formation of OH radical.For the Fe(III)/Cu(II)/H2O2 system, experimental data have demonstrated that the addition of Cu(II) increases the rate of decomposition of H2O2 (Figure 8a) and atrazine (Figure 8b) by Fe(III)/H2O2 and that these increases in reaction rates depend on the concentration of dissolved oxygen. This catalytic effect of Cu(II) has been attributed to a fast regeneration of Fe(II) (which is the major source of OH radical) by the reaction of Cu(I) with Fe(III). Since this reaction competes with oxidation of Cu(I) by O2 and H2O2, the catalytic properties of Fe(III) and Cu(II) mixtures will depend on the experimental conditions, such as the relative concentrations of reactants. In conclusion, this comparative study has confirmed that the rates of decomposition of H2O2 and atrazine, in dilute aqueous solution, by Fe(III)/Cu(II)/H2O2 are faster than by Fe(III)/H2O2 and Cu(II)/H2O2. This study has also demonstrated that dissolved oxygen has a significant effect on the reaction rates in the Cu(II)/H2O2 and Fe(III)/Cu(II)/H2O2.oxidation systems. The effects of dissolved oxygen and of the addition of Cu(II) on the efficiency of the Fe(III)/H2O2 system could be explained by assuming that the OH radical is the major oxidant species under our experimental conditions. However, additional research is needed in order to better understand the mechanism of decomposition of H2O2 by Cu(II) and Cu(I) and to determine the rate constants of individual reactions involved in the Cu(II)/H2O2 and Cu(I)/H2O2 systems

Oxydation de S-triazines par les procédés d'oxydation radicalaire. Sous-produits de réaction et constantes cinétiques de réaction

L'étude bibliographique montre que l'oxydation de l'atrazine en milieu aqueux par 03, 03/H202, 03/UV, H202/UV et TiO2/UV ne permet qu'une dégradation limitée du pesticide (pas d'ouverture de l'hétérocycle azoté). Ces procédés d'oxydation conduisent aux mêmes sous-produits d'oxydation. Les composés N-déalkylés, les acétamido-s-triazines et l'hydroxyatrazine constituent les premiers sous-produits de dégradation de l'atrazine. Une oxydation plus poussée conduit par des réactions de N-déalkylation, d'hydroxylation et de déamination à la formation de produits finals relativement stables comme la déséthyldésisopropylatrazine, l'amméline, l'ammélide et l'acide cyanurique. La distribution des différents sous-produits en cours d'oxydation dépend du procédé d'oxydation utilisé, des conditions de mise en oeuvre du procédé (dose d'oxydants ou d'UV, longueur d'onde d'irradiation,...), des caractéristiques des eaux de dilution (pH, pièges à radicaux hydroxyles,...).Les études cinétiques indiquent que l'atrazine est relativement réfractaire à une oxydation par l'ozone moléculaire (constante cinétique de l'ordre de 6 l mol-¹ s-¹ à 20 °C) et est assez réactive vis-à-vis des radicaux hydroxyles (constante cinétique de l'ordre de 2,5 10·9 mol-¹ s-¹ à 20 °C). En ce qui concerne les constantes cinétiques de réaction des radicaux hydroxyles sur les autres s-triazines, les résultats montrent que les méthylthio s-triazines sont beaucoup plus réactives que les méthoxy s-triazines qui sont elles mêmes légèrement plus réactives que les chloro et hydroxy s-triazines. Parmi les sous-produits d'oxydation de l'atrazine, la déséthyldésisopropylatrazine et l'acide cyanurique sont très réfractaires à une oxydation par les radicaux hydroxyles et par l'ozone moléculaire.In this paper, oxidation studies of s-triazines in aqueous solution by advanced oxidation processes (O3, O3/H2O2, O3/UV, H2O2/UV, et TiO2/UV) have been reviewed.Oxidation by-products of atrazine:Several investigators have shown that N-dealkylated (deethylatrazine and deisopropylatrazine) and acetamido-s-triazines are the primary oxidation by-products of atrazine by O3 and by O3/H2O2 (table 1; fig. 1a). Under conditions which favored the production of hydroxyl radicals (03/H2O2), trace amounts of hydroxyatrazine may also be formed. These primary by-products are subsequently degraded to give complete N-dealkylated, deamined, dehalogenated and hydroxylated s-triazines (deethyldeisopropylatrazine, ammelide ammeline, cyanuric acid,...) (table 1). For example, oxidation of deethylatrazine by O3/H2O2 yields deethyldeisopropylatrazine as the major by-product (fig. 1b). Identical by-products are produced by photochemical oxidation (O3/UV, H2O2/UV and TiO2/UV) (table 2; fig. 4 and 5). UV photolysis of atrazine at 253.7 nm (monochromatic radiation) yields hydroxyatrazine as the major product (=0.95 -1.0 mole of hydroxyatrazine formed / mole of atrazine photolysed (fig. 4a) whereas N and N,N'-dealkylated, deaminated and hydroxyderivatives are produced by UV irradiation in the presence of ozone, hydrogen peroxide (fig. 4b) or photosensitisers.The s-triazine ring is found to be resistant to chemical and photochemical oxidation. Pathways for the degradation of atrazine by molecular ozone (fig. 2) and by hydroxyl radical (fg. 3) are proposed.Kinetic rate constants:The second-order kinetic rate constants for the reaction of molecular ozone and of hydroxyl radical with atrazine have been determined by several authors from competitive experiments or from kinetic models The rate constants for the reaction of ozone which have been measured (~ 61 mol-¹ s-¹ at ~ 20°C) indicate that molecular ozone is not very reactive towards atrazine. The rate constants which have been determined for the reaction of hydroxyl radical with atrazine by using different modes of generation of hydroxyl radicals (O3 + OH-; O3 + H2O2; PhotoFenton; H2O2 + UV) are in the order of 2 10[exp]9 - 2.5 10[exp]9 l mol-¹ s-¹ at ~ 20°C (table 3).Rate constants for the reaction of hydroxyl radical with other s-triazines have been determined from competitive kinetic experiments. The relative rate constants show that methylthio s-triazines are far more reactive than methoxy s-triazines, which in turn are more reactive than chloro and hydroxy s-triazines (table 4). The kinetic data also confirm that deethyldeisopropylatrazine and cyanuric acid are very refractory to the oxidation by hydroxyl radical

Oxydation d'un acide humique aquatique par le bioxyde de chlore. Incidences sur une post-chloration et sur un traitement au charbon actif

Cette étude de laboratoire a eu pour but d'examiner la réactivité du bioxyde de chlore sur un acide humique d'origine aquatique en solution aqueuse et en milieu neutre (pH = 7,5) et de préciser en particulier l'incidence d'une préoxydation chimique au CIO2 sur les potentiels de formation de composés organohalogénés (trihalométhanes, acides dicloroacétique et trichloroacétique, chlore organiquement lié) et sur l'adsorbabilité du carbone organique sur charbon actif.Les résultats obtenus montrent que radian du bioxyde de chlore sur racide humique Pinail à l'obscurité, conduit à des faibles abattements du carbone organique dissous (< 10 %) et de l'absorbance UV à 254 nm (de l'ordre de 30 %) et conduit à des productions potentiel es en composées organohalogénés très nettement inférieures à celles formées par chloration. De plus, une préoxydation chimique au bioxyde de chlore permet de diminuer d'une manière très significative la production de composés organohalogénés au cours d'une post-chloration et semble améliorer l'adsorbabllité du carbone organique sur charbon actif.L'oxydation de l'acide humique par le bioxyde de chlore s'accompagne, par ailleurs, de la formation de chlorites (0,65 mg/mg de CIO2 consommé) qui peuvent ensuite être oxydés en chlorates au cours d'une post-chloration ou réduits en chlorures par un traitement au charbon actif.Enfin, les résultats obtenus font apparaître que le mécanisme d'oxydation de composés organiques parle bioxyde de chlore en présence de la lumière ainsi que les interactions entre le bioxyde de chore, les chlorites, la matière organique et le charbon actif méritent d'être plus précisément étudiés.Chlorine dioxide has drawn much recent attention as an alternative disinfectant and oxidant for drinking water to replace chlorine because of its powerful disinfecting ability and its limited capacity to produce organohalogenated compounds. However, the use of chlorine dioxide leads to chlorite (ClO2-) and chlorate (ClO3-) as inorganic oxidation by-products which are reported to have toxic effects on humans. The reactions of ClO2 with simple organic compounds (phenols, aliphatic and aromatic amines...) produce polar compounds such as quinone, ketones, aldehydes and carboxylic acids while oxydation by-products of dissolved organic matter of surface waters (in particular humic substances) are largely unknown. Consequently, the aim of this work was to obtain a better understanding of the effects of the use of chlorine dioxide in drinking water treatment To this end, experiments were carried out with dilute aqueous solutions of an isolated aquatic humic acid (Pinail humic acid, PHA) and the objectives of this present study were :- To evaluate the ClO2 demand and to determine the productions of chlorite, chlorate and of organohalogenated compounds such as trihalomethanes (THMs), dichloroacetic and trichloroacetic acids (DCA, TCA) which are the main organohalogenated products formed by chlorination.- To show the effects of chlorine dioxide preoxidation on organic halide formation potentials (postchlorination) and on the removal of dissolved organic carbon (DOC) by activated carbon. In addition, reactions of chlorite with chlorine or with activated carbon were also examined.EXPERIMENTALPniail humic acid was dissolved in phosphate buffered ultra-pure water (pH = 7.5). Oxidation and adsorption experiments were carried out in headspace-free bottles, at 20 ± 1 °C and in the dark. Stock solutions of chlorine dioxide (4-6 g l-1) and of chlorine (6-10 g l-1) were prepared in the laboratory and titrated by iodometry. Residual chlorine dioxide concentration in PHA solutions was determined by spectrophotometric measurement at 360 nm and by two colorimetric methods : the chlorophenol red and the ACVK methods. Concentrations of DOC and of total organic chlorine or halogen (TOCI, TOX) were measured using a DOHRMANN DC 80 carbon analyser and a DOHRMANN DX 20 A TOX analyser equipped with a microcoulometric cell, respectively. THMs, DCA and TCA were determined by a gas chromatograph equipped with a 63 Ni electron capture detector after extraction by pentane for the THMs, and methylation in ether phase for DCA and TCA. Inorganic chlorine species were analysed by HPLC with a UV detector (ClO2-) or by chromatography (Cl-, ClO3-).RESULTS• Oxidation of PHA by ClO2The results showed that PHA consumed about 2 mg of ClO2/mg of DOC after a reaction time of 24 hours (fig. 1) and that there is a rapid consumption of ClO2 during the first 30 minutes of the reaction (fig. 2) Oxidation by ClO2 had no effect on DOC concentration (DOC removal : < 10 %) and led to a significant decrease (about 30 %) of the UV-absorbance at 254 or 270 nm (fig. 1 and 2), and to productions of ClO2- (0,65 mg of ClO2-/mg of ClO2 consumed) which were independant of the applied oxidant dose and of the reaction time.Furthermore, after a 72 hour reaction time in the dark, chlorine dioxide ([ClO2]0 = 5 mg l-1, [PHA]0 = 5 mg l-1, DOC = 2,6 mg l-1) produces very small amounts of chloroform (< 5 µg l-1), DCA (5 µg l-1) and TCA (5 µg l-1) and organochlorinated compounds (TOCl : 36 µg/mg DOC) compared to chlorine oxidation (tableau 1). However, in the presence of sunlight, ClO2 is rapidly photodecomposed (fig. 3) and the photodegradation products of ClO2 allow bromide oxidation (fig. 11) and lead to higher productions of organohalogenated compounds such as THMs (fig. 4).• Chlorine dioxide preoxidation followed by chlorinationAs shown in figure 5, chlorine dioxide preoxidation reduces the production of organohalogenated compounds and the chlorine demand during postchlorination. For a preoxidant dose corresponding to the ClO2 demand of PHA, the decrease in the formation potentials of CHCl3, DCA, TCA and TOCl was about 40-50 %. These results confirm the similarity of the action of chlorine dioxide and chlorine on aromatic structures which have high electron density carbons and which constitute probably the most reactive precursors of organohalogenated by-products.As far as chlorite concentration is concerned, the results showed that chlorite formed during the preoxidation step was completely oxidized to chlorate during postchlorination, under the experimental conditions used in this study (chlorine dose : 40 mg l-1; contact time : 24 or 72 hours). Because of the reactions of chlorine eh chlorine and with residual chlorine dioxide, a small increase in the chlorine demand was observed when PHA solutions were heavily preoxidized (fig. 5).• Chlorine dioxide preoxidation followed by activated carbon treatmentBatch experiments were carried out with a powdered activated carbon (PAC, granulometry : < 80 µm) which was obtained by crushing a commercial granular activated carbon (CECA 40,12 x 40 mesh). Once equilibrium was achieved (contact time : 10 days), adsorption isotherms indicated that chlorine dioxide preoxidation increases the absorbability of DOC on activated carbon (fig; 4tableau 2). Furthermore, chlorite in oxidized PHA solutions was reduced by PAC to chloride. The capacity of CECA 40 activated carbon for ClO2- reduction to Cl- was about 170 mg ClO2-/g of PAC (fig. 7). Other experiments showed that chlorite may react with specific surface groups on PAC to produce inorganic carbon (fig. 7) and with PHA only in the presence of PAC as shown the DOC and UV-absorbance curves in figure 8 and the increase of TOX concentration in the liquid phase in figure 9. Thus the observed increase in DOC absorbability on PAC after a chlorine dioxide preoxidation may be attributed to cheminal interactions between PAC, chlorite, residual chlorine dioxide and adsorbed organic matter and requires further study

Value creation:What matters most in Communities of Learning Practice in higher education

This study examines the phenomenon of value creation enabled by peers’ voluntary participation in Communities of Learning Practice (CoLPs) in higher education, with the aim to extract which experiences of learning community participation are considered valuable by learning community members. The participants were 27 international master students at a German university. Data were collected from participants’ written narratives-so called value creation stories. A systematic qualitative research approach was employed. Initially, we conducted a theory-driven content analysis to classify members’ attributed values. Subsequently, we performed an emergent data-driven thematic analysis to extrapolate the specifics of attributed values by participants. This study underscores the role of learning community members’ agency in value creation, by having community members, instead of external members, define value creation for themselves, as an individual and collective process and “outcome” enabled by participation in CoLPs.<br/

Etude de la dégradation de quelques composés organochlorés volatils par photolyse du peroxyde d'hydrogène en milieux aqueux

Le travail a eu pour but d'étudier l'efficacité de la photolyse du peroxyde d'hydrogène sur la dégradation de quelques composés organochlorés aliphatiques saturés (chiorométhanes et chloroéthanes) en milieu aqueux (pH 7,5). Les expériences ont été réalisées en réacteur statique, avec une Lampe basse pression à vapeur de mercure et avec des concentrations initiales en produit chloré de l'ordre de 10-6 mol l-1 et en H202 comprises entre 10-5 et 10-3 mol L-.Les résultats montrent que le système H202/UV peut oxyder les composés organochlorés étudiés à l'exception des composés ne possédant pas d'atome d'hydrogène (CCL4 et C2 CL6). Les rendements d'oxydation obtenus avec Le réacteur utilisé dépendent du temps de réaction, de la concentration initiale en H202, du flux photonique et peuvent être nettement diminués par la présence de pièges à radicaux (ions bicarbonates) dans le milieu réactionnel.Par ailleurs, une étude cinétique de la photolyse du peroxyde d'hydrogène en absence de matière organique est également présentée.The aim of this work was to study oxidation of certain volatile polychlorinated hydrocarbons, using hydrogen peroxide photoactivated by UV. This research was carried out with different mixtures of diluted aqueous solutions of chloromethanes (CHCl3, CCl4) and chloroethanes (C2H3Cl3, C2H2Cl4, C2HCl5, CCl6), which are typical halogenated compounds most frequently found in contaminated groundwater. The effect of the hydrogen peroxide concentration, the light intensity and the bicarbonate concentration on the rate of 1,1,2-trichloroethane (TCE) oxidation was determined. A kinetic study on hydrogen peroxide photolysis in a solution free of organic compounds was also carried out.EXPERIMENTATIONExperiments were conducted in a batch reactor (V = 4 l), equipped with an immersed mercury low-pressure lamp. The intensity emitted at 253.7 nm was roughly 2 1019 photons s-1. The temperature of the reaction mixture was maintained with a regulation system at 16 ± 0.5 °C (figure 1).The solutions were prepared in a phosphate buffer µ = 2 10-2 M, pH = 7.5). The outer surface of the lamp was masked with strips of aluminium, so as to obtain various percentages of initial energy (20 to 100 %).The concentration of the hydrogen peroxide of the samples was determined by spectrophotometry and the chlorinated compounds were analysed by electron capture gas chromatography.RESULTKinetics of hydrogen peroxide photolysis : H202 was decomposed by UV tb produce two hydroxyl radicals. In diluted solutions ([H202] < 10-3 M), the concentration decreases in accordante with a first order law. The rate constant depends on the initial light intensity (Io), on the characteristics of the reactor (volume and distance between the lamp and the watt. of the reactor) and on the motar extinction coefficient of the irradiated solution (equation C). The decomposition rate appears to be dependent on pH, the rate of constant rire has been found to be proportional to the dissociation of hydrogen peroxide into its basic form (EH2O2 = 20 mol-1 cm-1,EH2O2_ = 240 mol-1 cm-1) (figure 2 and 3).Oxidation of the chlorinated compounds : H202/UV is very efficient for the removal of organic compounds. Preliminary experiments showed that both UV and H2O2 treatments do not decompose halogenated compounds. Hydroxyl radicals are extremly reactive and attack organic compounds preferentialty by abs-tracting a hydrogen atom from an organic molecule. This is confirmed by the results which show that chloromethanes and chloroethanes with an H atom are eliminated, but net compounds such as tetrachloride and hexachloroethane (figure 4 to 7).The effectiveness of an H202/UV system depends on various parameters. Studies on the TCE elimination show that the oxidation yields an increase when the reaction time, the UV irradiation dose (figure 9b) and the hydrogen peroxide concentration (figure 8) increase. However, the efficiency decreases in the presence of radical traps such as bicarbonate and carbonate ions (figure 10)

How can contributors to open-source communities be trusted? On the assumption, inference, and substitution of trust

Open-source communities that focus on content rely squarely on the contributions of invisible strangers in cyberspace. How do such communities handle the problem of trusting that strangers have good intentions and adequate competence? This question is explored in relation to communities in which such trust is a vital issue: peer production of software (FreeBSD and Mozilla in particular) and encyclopaedia entries (Wikipedia in particular). In the context of open-source software, it is argued that trust was inferred from an underlying 'hacker ethic', which already existed. The Wikipedian project, by contrast, had to create an appropriate ethic along the way. In the interim, the assumption simply had to be that potential contributors were trustworthy; they were granted 'substantial trust'. Subsequently, projects from both communities introduced rules and regulations which partly substituted for the need to perceive contributors as trustworthy. They faced a design choice in the continuum between a high-discretion design (granting a large amount of trust to contributors) and a low-discretion design (leaving only a small amount of trust to contributors). It is found that open-source designs for software and encyclopaedias are likely to converge in the future towards a mid-level of discretion. In such a design the anonymous user is no longer invested with unquestioning trust